Vacuum packaging (generally deep draw packaging or skin packaging) is employed for packaging of objects (in particular, processed foods such as processed meat products and fish-based paste products).
In the case of deep draw packaging, generally, a belt-shaped film is continuously fed to a cooling mold having a plurality of concave portions; the film is heated, and then pressed onto the mold under vacuum and/or pressurized conditions, thereby forming a plurality of concave containers through deep drawing; an object (e.g., sliced ham) is placed in each of the containers; a film for a top film is continuously fed to cover the containers; and air is evacuated from the containers, followed by thermal sealing of the upper peripheral portion of each of the containers with the top film, to thereby produce a plurality of vacuum-packaged products in a continuous manner.
However, deep draw packaging involves problems in that, for example, wrinkles are formed at a portion at which a top film or an object (e.g., roast pork) placed in a concave container comes into close contact with the side(s) or bottom of the concave container, vacant gaps are formed at the contact portion and meat juice or drip enters the vacant gaps, or pinholes tend to be generated at the thus-formed wrinkles or vacant gaps. This occurs because a variation in the size of the object as measured in a horizontal direction is greater than a variation in the size thereof as measured in a vertical direction. Therefore, a mold employed in deep draw packaging is generally required to have a size and a shape which fit those of an object to be packaged.
In food processing manufacturers, in pursuit of high production efficiency, a variety of objects have been vacuum-packaged through deep draw packaging by use of a general-purpose mold (without exchange of the mold). However, there have arisen more and more quality-related problems caused by the aforementioned wrinkles, vacant gaps, or pinholes.
In view of the foregoing, there has been disclosed a method in which an object is vacuum-packaged with a conventional multi-layer film for deep draw packaging, and high-temperature steam is sprayed onto the resultant packaged product for thermal shrinkage of the film, to thereby smooth out wrinkles from being formed on the packaged product. However, this method involves problems in that wrinkles may be newly formed at a portion of the packaged product that has no wrinkles, as well as at a portion thereof that has wrinkles, and that the contents of the packaged product may collapse as a result of shrinkage of the entirety of the film.
The aforementioned method employing steam spraying, or a method in which a packaged product is boiled so as to thermally treat the contents of the product and to smooth out wrinkles (see claim 1 and sections 0024 and 0025 of Japanese Patent Application Laid-Open (kokai) No. 8-198208) involves problems in terms of moisture remaining in the packaged product or removal of the moisture.
Meanwhile, there has been disclosed an apparatus for smoothing out wrinkles on a deep draw vacuum-packaged product, which apparatus includes a hot air generation/spraying unit for spraying hot air having a temperature equal to or higher than the molding temperature onto only wrinkles formed on the surface of a film serving as a bottom film for the container portion of a vacuum-packaged product (hereinafter such a film may be referred to as a “container material film”); a unit for cooling the container material film for preventing the effect of hot air on a portion of the film that has no wrinkles; a flange portion cooling unit for preventing the effect of hot air on a flange portion of the packaged product; and a hot air jet shielding unit for preventing the effect of hot air on other units and on packaged products adjacent to the target vacuum-packaged product (see claim 1 of Japanese Patent Application Laid-Open (kokai) No. 4-31218).
However, in the case of this technique, when an object to be packaged has a shape and a size which considerably differ from those of a mold, a portion of a packaging film that undergoes shrinkage may vary depending on the type of the unit to be employed, or the film may fail to be shrunk sufficiently, and therefore, wrinkles cannot be smoothed out completely. In the case where a shrinkable film is employed, a peripheral seal portion (non-molded portion) of the container portion of a packaged product also undergoes shrinkage, and thus the seal portion is curled attributed to the difference in percent shrinkage between the seal portion and a top film, leading to poor appearance of the product.
Such a container material film employed for deep draw packaging is required to have, for example, deep draw moldability, heat resistance, pinhole resistance, labelability, and sealability to a top film. In addition, the film is required to have gloss, transparency, sealing strength, and ability to prevent removal of a label therefrom. In accordance with the type of an object to be packaged, the film may be required to have easy peelability to a top film. In order to satisfy such requirements, a variety of packaging materials have been proposed.
For example, there has been disclosed a co-extruded composite film for deep draw molding, the film including a polybutylene terephthalate resin layer serving as the outermost layer, a sealable resin layer serving as the innermost layer, and a gas-barrier resin (e.g., polyamide resin) layer serving as an intermediate layer (see claim 1 of Japanese Patent Application Laid-Open (kokai) No. 6-226930).
Also, there has been disclosed a method employing, as a container material film, a stretch-oriented multi-layer film which includes a surface layer formed of a thermoplastic resin, an intermediate layer formed of a polyamide resin, and a surface layer formed of a sealable resin, and which has an impact energy (on the basis of a 50 μm thickness) of 1.5 J or more as measured at −10° C., wherein a packaged product is produced under vacuum by use of a conventional deep draw packaging apparatus, and then the product is subjected to shrinkage with hot water (e.g., boiling) for smoothing out wrinkles thereon (see claim 1 of International Patent Publication WO 01/98081).
Meanwhile, skin packaging is advantageous in that a film for packaging can be brought into close contact with an object to be packaged, and thus no wrinkles are formed on the resultant packaged product; i.e., the packaged product exhibits good appearance. In a fundamental technique of skin packaging, an object to be packaged is placed on a base film, a heat-softened film for skin packaging is covered over the object, and air is evacuated from the resultant product, to thereby bring the skin-packaging film into close contact with the object.
There has been disclosed a skin packaging method in which an object is packaged, by use of a skin packaging apparatus, with a skin-packaging film which includes at least two layers: a sealable resin layer (a) constituting the side of the film opposite the side which faces a mold, and a stretched polyamide resin layer (b) formed of a polyamide resin having a melting point higher, by about 15° C. or more, than the heating temperature of the film, and which exhibits, at the film heating temperature, a thermal shrinkage stress of 200 g/15-mm width or less and a percent thermal shrinkage of 3% or more as measured in at least one direction (see claims 1 through 7 of Japanese Patent Application Laid-Open (kokai) No. 2001-71435).
This publication describes that the packaging method can produce a wrinkle-free skin-packaged product. However, this method fails to solve economical problems caused by low molding speed (i.e., a disadvantage derived from skin packaging).